A prospective clinical study was undertaken to investigate possible structural alterations in the livers of patients exposed to halogenated inhalational anesthetics during surgery. Patients undergoing elective staging laparotomies with liver biopsies were studied. Only patients free of gross hepatic pathology were included in the analysis of data. Patients were anesthetized and maintained on N2O-O2-narcotic until the laparotomy incision was completed and a first liver biopsy ("pre-anesthesia") obtained. Patients were then changed to maintenance anesthesia with a halogenated inhalational agent, either halotane or fluroxene. After 60 minutes or more exposure to the halogenated agent a second liver biopsy ("post- anesthesia") was taken. Control patients received no halogenated anesthetic. Pre- and post-anesthesia biopsies were examined for ultrastructural indications of toxic injury with the electron microscope. Analysis of 13 patients has been completed. Three control patients showed no hepatocyte structural alterations between the pre- and post- anesthesia biopsies. By contrast all 10 patients exposed to halogenated anesthetics showed evidence of toxic cellular injury developing between the pre- and post-anesthesia biopsies. Three patients given fluroxene showed increases in intracellular lipid. Seven halothane patients exhibited dilation of smooth endoplasmic reticulum; four of the seven showed lipid accumulation.